Ultrasonic flow measuring devices are used frequently in process and automation technology. They enable a contactless determination of volume, and/or mass, flow of a medium in a pipeline.
Known ultrasonic flow measuring devices work either according to the Doppler principle or according to the travel time difference principle. In the case of the travel time difference principle, the difference in travel times of ultrasonic measuring signals in the stream direction and counter to the stream direction of the medium is evaluated. For this, the ultrasonic measuring signals are alternately emitted from, and received by, the ultrasonic sensors in the stream direction and counter to the stream direction of the medium. From the travel time difference of the ultrasonic measuring signals, the flow velocity can be determined, and, therewith, in the case of known diameter of the pipeline, the volume flow rate, and, in the case of known, or measured, density of the medium, the mass flow rate.
In the case of the Doppler principle, ultrasonic measuring signals of predetermined frequency are coupled into the flowing medium. The ultrasonic measuring signals reflected in the medium are evaluated on the basis of the frequency shift arising between the in-coupled and the reflected ultrasonic measuring signals. Here also, the flow velocity, or the volume, and/or mass, flow rate of the medium can be determined.
Use of flow measuring devices working according to the Doppler principle is only possible when the medium contains air bubbles or impurities, on which the ultrasonic measuring signals can be reflected. Therefore, the use of such ultrasonic flow measuring devices is rather limited in comparison to ultrasonic flow measuring devices working according to the travel time difference principle.
Regarding measuring device types, a distinction is drawn between ultrasonic measurement pickups, which are inserted into the pipeline, and clamp-on flow measuring devices, where the ultrasonic sensors are pressed externally onto the pipeline by means of a clamping mechanism. Clamp-on flow measuring devices are described, for example, in EP 0 686 255 B1, U.S. Pat. No. 4,484,478 or U.S. Pat. No. 4,598,593.
In the case of both types of ultrasonic flow measuring devices, the ultrasonic measuring signals are, at a predetermined angle, radiated into, respectively received from, the pipeline, or measuring tube, as the case may be, in which the flowing medium is located. In order to achieve an optimum impedance matching, the ultrasonic measuring signals are coupled into, respectively out of, the pipeline via a mediating body, e.g. a coupling wedge. The main component of an ultrasonic sensor is at least one piezoelectric element, which produces and/or receives the ultrasonic measuring signals.
Now, an ultrasonic sensor produces, besides the actual wanted-signal serving for determining the volume, and/or mass, flow, also a so called body signal, i.e. a certain fraction of the ultrasonic measuring signal propagates via the wall of the pipeline and superimposes as a disturbance signal on the actual flow-measuring signal. As a counter measure, it has long been a practice to select the sound path through the pipeline, respectively through the medium being measured, to be as long as possible, such that the travel time of the measuring signal crossing the medium clearly differs from the travel time of the signal propagating via the wall of the pipeline. Problems do, however, arise when the pipeline has a small diameter. In this case, it is difficult to avoid errors in the measurement. Consequently, it would be very advantageous to prevent these disturbance signals, which propagate via the tube wall, already at the source and not only later to attempt to filter or eliminate them from the measured signals.